The present invention relates to a lockable freewheeling device and, more particularly, to a device which provides a driving connection between non-permanently driven wheels of the rear axle and permanently driven wheels of the front axle of a motor vehicle.
The device includes two freewheeling parts. One of the freewheeling parts is driven and the other one is intended to pass on the rotational movement. One of the freewheeling parts includes a sleeve-shaped portion with circumferentially distributed apertures holding locking members in a radially displaceable way. The other freewheeling part includes circumferentially distributed recesses which correspond to the apertures. Also, the sleeve-shaped portion has an axially displaceable switching element. The switching element is pushed by a spring into a position in which the locking members are held in the recesses. Further, as a function of the speed, the switching element may be moved into a released position, via a switching device against the force of the spring. The switching device includes a switching cage and centrifugal masses which are radially displaceable as a function of the speed. The masses are supported on supporting faces of one of the freewheeling parts and the switching cage. Also, the masses are inclined relative to one another such that, under the effect of the centrifugal force, the switching cage is axially adjusted relative to the freewheeling parts.
Freewheeling devices are illustrated in DE-4027209.Cl, issued Aug. 28, 1990 and in the unpublished patent application 41 36 271.3, issued Nov. 4, 1991. U.S. Pat. No. 4,889,353 describes a state of the art drive assembly for a four wheel drive vehicle. The assembly has a viscous coupling which, via a freewheeling device, is either connected to, or disconnected from, the input end of the non-permanently driven wheels. Also, the assembly has two clamping member freewheeling units which are engaged up to a predetermined speed. The freewheeling units provide a non-rotating connection for both directions of rotation. Once a certain speed is exceeded, the clamping members of the clamping member freewheeling unit, which serves to transmit torque during reversing, are made ineffective. This ensures that if the rear wheels rotate faster than the front wheels, for example during braking, overtaking is possible. In this way, it is ensured that no braking moment is transmitted from the front wheels to the rear wheels and that even when the front wheels are overbraked, lateral stability of the vehicle is maintained.
It is known to use viscous couplings to connect non-permanently driven vehicle wheels. GB 1,357,106, issued Jun. 19, 1974 is such an example. The viscous coupling is arranged in the driveline between the front and rear wheels and it reacts to a speed differential between the front and rear wheels.
Viscous couplings used as so-called torque splitters, form another part of the state of the art, in which case they replace the rear wheel differential of the non-driven axle of a four wheel drive vehicle. Such an assembly is described in DE 37 08 193 A1, issued Oct. 1, 1987. This publication also describes freewheeling units which, during the driving operation, have a locking effect in both directions of rotation, but which release upon operation of the vehicle brake.
In the case of freewheeling units switched as a function of speed, e.g. freewheeling units switched via centrifugal force, the connecting speed may be passed through in the course of driving. However, when braking, on a smooth road surface, considerable speed differentials may occur between the axles, e.g. the front axle may have a locking effect. If the locked condition is eliminated by releasing the brake for example, the freewheeling unit again moves into the released position. If such resulting switching operations are repeated, it is possible for a switching noise to occur. Furthermore, such sudden changes may result in damage.